Download App Note 507 - Charging Large Load Capacitors

App Note 507 - Charging Large Load Capacitors
Introduction
Lambda’s ALE series Capacitor Charging Power Supplies, are
specifically designed to rapidly and efficiently charge capacitors
in pulsed discharge loads such as lasers and modulators. These
supplies operate as constant current sources which makes them
ideal for operating with the variable load impedance of a charging
capacitor. This application note aims to highlight the advantages
of the ALE series supplies, and some useful precautions when
charging large high energy storage capacitors.
The LCA module requires no modification to the normal control
circuits and operates in both remote and local mode.
Calculating load charge time with an LCA equipped power supply
involves a charge simulation spreadsheet that can be obtained
from our web site, or by contacting the factory. The simulation
requires the load circuit information to be defined (Capacitance,
Charge Voltage, Supply Rated Voltage), and allows the user to
try different power supply combinations for optimum circuit operation.
Load Fault Condition
Example
ALE power supplies are rated to run in repetitive circuits that often
operate with tens or hundreds of charge/discharge cycles per second. In this case capacitor charge times are short and the power
supply reaches the programmed output voltage typically within a
few milliseconds or tens of milliseconds.
The following example demonstrates a standard 30kW, 26kV
HVDC power supply, and a capacitor charging supply with and
without the LCA, charging a 7000µF load capacitor to 24kV.
When supplies are used to charge large loads over a few seconds
or longer, the power supply will indicate a Load Fault or Overload
condition. Load Fault is a simple timer circuit within the supply
designed to protect it and the load, in the event of an external
short or latching condition. The supply output shuts down (‘off’
state) and indicates a Load Fault if the output voltage does not
reach the programmed voltage after charging for 500ms. Following 500ms in the ‘off’ state, the load fault indication clears and the
supply automatically begins recharging the load(1). This 500ms on,
500ms off cycle continues until the programmed output voltage is
reached, leading to a staircase like charge voltage waveform.
Conventional HVDC supply
A conventional HVDC power supply with a 30kW, 26kV rating has
an output current rating of 1.15A (30kW/26kV). The time to charge
7000µF to 24kV is given by;
Tc = C×V/I = 7000×10-6 × 24×103 / 1.15 = 146seconds
Capacitor Charging Supply without LCA
The ALE model 303 is a 30kW rated capacitor charging supply
with a peak output current of 2.88A at 26kV. The average charging current is half this value (1.44A) when the supply operates in
load fault mode. The charge time is given by;
Tc = C×V/I = 7000×10-6 × 24×103 / 1.44 = 117seconds
Operating the power supply in this mode will not cause any damage to the unit, but it is not the fastest way to charge the load since
the supply is only operating at 50% duty cycle.
Capacitor Charging Supply with LCA
Using the same model ALE 303 supply, and the Long Charge
Adapter, the time to charge the load to 24kV is;
Long Charge Adapter
To realize the fastest charge time from any ALE supply charging large energy storage capacitors, a programming module is
available that optimizes the output current profile of the supply.
Using this module results in significantly improved charge times
compared with conventional HVDC power supplies with identical
power ratings, or capacitor charging supplies operated in the Load
Fault mode.
The graph below shows the charge waveforms for each case of
the example above.
Tc = 66seconds (using simulation spreadsheet)
The ALE Long Charge Adapter (or LCA) is a simple module that
plugs into the power supply remote control interface and modifies
its output current to automatically minimize charge time for large
loads. A sketch of the LCA is shown below.
2
3
1
CH1 4.00kV
CH3 4.00kV
CH2 4.00kV
M 15.0s
Legend
With the LCA installed the Load Fault circuit is effectively defeated, allowing the supply to continuously deliver its full rated charge
current while the output voltage is less than 50% of rated, and
linearly reduces output current to half its rated value at 100% of
rated voltage.
CH1 = 303-26kV with LCA - Tc = 66secs
CH2 = 303-26kV w/o LCA - Tc = 117secs
CH3 = Conventional 30kW 26kV HVDC supply - Tc = 146secs.
The graph and data clearly shows that the ALE 303 supply
equipped with the LCA, and with the same rating as a conventional 30kW DC supply, charges the 7000µF load to 24kV in less
than half the time.
Note 1. If LP option is enabled, supply requires HV ON/OFF cycle to clear Load Fault.
TDK-Lambda Americas Inc., High Power Division,
405 Essex Road, Neptune NJ 07753 USA
T: +1-732-922-9300 F: +1-732-922-1441
App Note 507 - Charging Large Load Capacitors (continued)
The faster charge time significantly reduces voltage stress on the
load capacitor resulting in a longer operating life.
There are three different LCAs available, each for use with one of
three product families. All LCAs function in an identical manner,
but have different connector pin arrangements depending on the
power supply family control interface.
Part 26922100 for models 500A, 102A, 152A, and 202A
Part 26922200 for models 402, 802, XR802, and LC1202
Part 26922300 for models 203 and 303
The LCA can be purchased with the supply at the time of ordering,
or as a spare part through our repair depot, contact the factory for
current pricing.
Safety Precautions
When a capacitor charging power supply is used to charge a load
circuit that contains greater that 1kiloJoule of stored energy, it
is wise to add an external isolation network between the power
supply and the load. The isolation circuit will prevent the load capacitor from discharging into the power supply in the event of a
catastrophic failure in the output section of the supply. The power
supply warranty may be voided if an isolation network is not installed. The sketch below shows the recommended isolation network.
HV Components
PO Box848, Farmingdale, NJ 07727 USA
Tel. 732-938-4499
Web: www.hvca.com
Email: [email protected]
Voltage Multipliers Inc
8711 W. Roosevelt Ave, Visalia, CA. 93291 USA.
Tel. 559-651-1402.
Web: www.voltagemultipliers.com
High Energy Resistors manufacturers
Kanthal Globar
495 Commerce Drive, Amherst, NY 14228 USA
Tel. 716-691-4010
Web: www.globar.com
HVR Advanced Power Components
1307 Military Rd, Tonawanda, NY 14217 USA
Tel. 716-693-4700
Web: www.hvrapc.com
Ri
High Voltage Relay manufacturers
Di
Ross Engineering Corporation
540 Westchester Drive, Campbell, CA 95008 USA
Tel. 800-654-3205
Web: www.rossengineeringcorp.com/hv_relays.htm
Power
Supply
High Voltage Diode manufacturers
Cload
Tyco Electronics (Kilovac)
Tel. 800-253-4560
Web: relays.tycoelectronics.com/kilovac/
Diode Di isolates the power supply from the load and in the event
of a catastrophic failure in the power supply output section will
prevent rapid discharge of the energy in Cload through the supply,
which could present a safety hazard. Di should have a reverse
voltage rating at least 1.5 times the rating of the power supply,
and a forward current rating at least 2 times the power supply
capability.
If you have any questions or comments regarding
this or any of our Application Notes or products,
please contact Andy Tydeman at the factory, we
are here to help.
Information cannot be guaranteed and may be subject to change without notice.
Resistor Ri is designed to dissipate the energy stored in Cload in
the event of a power supply output failure. The value of Ri should
be approximately 100Ω with an energy rating sufficient to dissipate all of the stored energy in Cload.
For additional isolation it is recommended that the power supply
is disconnected from the load circuit using a high voltage relay or
disconnect switch prior to load discharge.
For suggested protection circuit component manufacturers, see
opposite.
© 2009 TDK-Lambda Americas Inc.
TDK-Lambda Americas Inc., High Power Division
405 Essex Road, Neptune NJ 07753 USA
T: +1-732-922-9300 F: +1-732-922-1441
93008507. Rev D.